1. Field of the Invention
The present invention relates generally to active noise suppression. More particularly, this invention relates to a multi-channel active control system and method for the reduction of tonal noise from an axial fan.
2. Related Art
Axial fans are air-moving devices in which the air flows parallel to the axis of rotation. One common use of these fans is in the cooling of electronic equipment, such as computers, copiers, overhead projectors, and related technology, where the fan is most often mounted on one face of the equipment enclosure. The noise generated from the fans on these devices can contribute significantly to background noise levels in the workplace, classroom, and anywhere else such fans are located, causing annoyance and disrupting concentration to persons that can hear the noise. The radiated noise is composed of both broadband and tonal components. The tonal components are referred to herein as “tones.” The tones may rise as much as 25-30 decibels (dB) above broadband levels, and therefore typically dominate an axial fan's overall spectrum.
The tones most typically present in an axial fan's radiated spectrum are the blade passage frequency (BPF), calculated by multiplying the number of fan blades by the shaft speed in revolutions per second, and several harmonics, i.e., integer multiples of the BPF. The tones are typically caused by obstructions near the inlet or the outlet of the fan, such as fan supports, wires, or finger guards. Because the tones are usually the most dominant and noticeable part of the fan's spectrum, it is therefore desirable to reduce their levels or eliminate them entirely.
Techniques relating to fan noise reduction can be categorized as either passive or active. Passive noise reduction techniques typically use some sort of sound absorbing or muffling device. Whereas active noise reduction techniques typically involve the generation of an acoustic signal through a control source designed to destructively interfere with and therefore reduce the fan's acoustic radiation. Passive techniques have been limited in their effectiveness for two reasons: (1) the tonal content of the fan's radiation is often relatively low in frequency, such that a muffler or absorber would have to be made impractically large to fulfill its purpose and (2) the demand for reduction in size of technology makes the removal of obstructions to the flow of air near the inlet and outlet of the fan difficult. Because of the ineffectiveness of passive noise reduction techniques, conventional approaches disclosed over the past decade have primarily investigated the use of active noise control (ANC) to reduce the tonal noise from axial fans.
ANC has been applied to large fans, such as turbofan engines for commercial aircraft. In these larger turbofans, tonal noise is principally caused by the rotor-stator interaction, see for example, Thomas et al., “Active Control of Fan Noise from a Turbofan Engine,” AIAA JOURNAL, Vol. 32, No. 1, pp. 23-30, January 1994. Thomas et al: disclosed tests on an executive jet-type turbofan engine with a ring of actuators surrounding the fan. The error sensors were located in the far field outside the engine duct. Some limited on-axis far field control was achieved. Carl H. Gerhold, “Active Control of Fan-Generated Tone Noise,” AIAA JOURNALm, Vol. 35, No. 1, pp. 17-22, January 1997, discloses in-duct error sensors in order to improve on the effort by Thomas et al. with a ducted fan meant to simulate an engine environment. Gerhold disclosed that the new sensor location helped reduce acoustic spillover (increases in pressure levels) at large angles relative to the duct's axis, increasing the potential for obtaining global control of fan tones.
ANC has also been applied to a centrifugal fan or blower, such as those used in HVAC systems, see for example, Koopmann et al. “Active Source Cancellation of the Blade Tone Fundamental and Harmonics in Centrifugal Fans,” JOURNAL OF SOUND AND VIBRATION, 126(2) pp. 209-220, 1988. Koopmann et al. achieved duct inlet and outlet noise reduction for a fan's first two harmonics of a ducted centrifugal fan having two control sources located within one-quarter wavelength of the aerodynamic source.
D. A. Quinlan, “Application of Active Control to Axial Flow Fans”, NOISE CONTROL ENGINEERING JOURNAL, Vol. 39, No. 3, pp. 95-101, November-December, 1992, discloses a single channel control system in a research setting. The Quinlan apparatus included a single loudspeaker and error microphone. Since then, several variations of this single channel system have been proposed and investigated, see for example, Lauchle et al., “Active Control of Axial-flow Fan Noise,” JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, 101 (1), pp. 341-49 (1997); Wu, “Active Cancellation of Small Cooling Fan Noise from Office Equipment,” PROCEEDINGS OF INTER-NOISE 95, Newport Beach, Calif. (Noise Control Foundation, Poughkeepsie, N.Y., 1995), pp. 525-28; U.S. Pat. No. 6,188,770 B1 to Okada and U.S. Pat. No. 5,448,645 to Guerci. There are several deficiencies with the above conventional approaches that make wide-scale implementation of the proposed systems both unsuitable and impractical.
First, the single channel systems disclosed in Quinlan, Lauchle et al. and Wu demonstrate an inability to significantly attenuate on a global scale more than the BPF and second harmonic, whereas, higher harmonics are often present and their reduction desirable. Second, Lauchle et al. and Wu disclose error microphones placed in the fan's acoustic near-field, a virtual necessity in a practical implementation of such a control system, where the error sensor used would have to be located in or on the equipment enclosure. However, there is no evidence that the methods disclosed in Lauchle et al. and Wu for determining the appropriate near-field microphone locations could ever be implemented on a wide-scale basis.
Okada discloses a system similar to Quinlan, though there are several variations on nonacoustic sensors disclosed. Like the other single channel systems disclosed, the limitations of Okada's system include: (1) only a single actuator and microphone are used, which is insufficient for higher harmonics of the BPF, (2) the diagram (FIG. 7 of Okada) of the adaptive system described shows the orientation of the loudspeaker along a plane parallel to the axis of the fan, which could provide a mismatch in the sources' directivities at higher harmonics and, (3) there is no indication of the error microphone being placed in the acoustic near-field of the fan, nor is there any mention made of how the microphone placement will result in global attenuations of the fan's BPF and its harmonics.
Guerci exhibits fundamental physical inconsistencies, which would likely limit the system's ability ot globally attenuate a fan's BPF over extended periods of time. Guerci intended to eliminate the need for a digitally-based adaptive controller by using a microphone and a band pass filter to sense the frequency to be cancelled and then manually adjusting the amplitude and phasing of an array of control loudspeakers to attenuate the tone. There are differences of varialbe speed fans, there is no provision for the variations in amplitude and phase of the fan's radiation due to changes of loading on the fan over time.
The Guerci system requires constant readjustment for changes in amplitude and phase. In addition, the amplitude of the fan noise will vary as a function of frequency. There is no provision made in Guerci for adjusting loudspeaker amplitudes at different fan speeds. Furthermore, Guerci's band pass filter, which extracts the harmonic noise from the microphone's signal, has a center frequency equal to the fan's BPF that is manually adjusted with a potentiometer. With fluctuations in the fan's BPF due to airflow changes, the filter's center frequency would have to be frequently readjusted. Finally, there is no provision for dealing with harmonics of the BPF, which are often noticeable to the observer, even when the fundamental has been attenuated. Accordingly, there exists a need in the art for a multi-channel active control system and method for the reduction of tonal noise from axial fans.